1. Field of the Invention
The present invention relates to microchannel plate image intensifiers (MCPIs) and more specifically, to the use of a collimator to improve the resolution of proximity-focused MCPIs.
2. Description of Related Art
Image intensifier tubes are electro-optical devices which are used to detect, intensify and shutter optical images from the near ultraviolet to the near infrared regions of the electromagnetic spectrum. They are used for intensifying weak images for night vision and night blindness, for astronomy, electron microscopy, medical research, radiology, and as high-speed light shutters. Image tubes are also used for intensifying an image and as "active" light shuttering devices, permitting very short exposure times.
A proximity focused, MCP intensifier tube consists of an evacuated enclosure containing an image sensor (a photocathode) for conversion of an incident radiant image to a low-energy electron image, a proximity-focusing electron lens for focusing the electron image, a microchannel plate (MCP) for amplifying the electron image current, a second proximity focusing lens and a phosphor screen for conversion of the electron image to a light image.
It is estimated that about 20% of the electrons from the cathode are elastically scattered when they hit the MCP input surface. They rebound, are repelled by the cathode-to-MCP electric field and strike the MCP surface a second time at a distance of up to twice the cathode-to-MCP spacing from the first impact, or within a circle of about 800 microns diameter on the MCP input surface. In the screen region the same phenomenon occurs, but the spacing is about 1.2 mm so that the circle diameter on the screen is about 5 mm. With 20% of the electrons from an initial spot size of 50 microns, for example, distributed in some fashion over a 20 mm square area, the density is fairly low. In fact, the spot spreading effect is seen at amplitudes of about three orders of magnitude below the peak. This results in crosstalk, which becomes important when a bright signal is located adjacent to a weak signal, as when spatially multiplexing several inputs on a streak camera cathode.
The intensifier tube uses a microchannel plate for internal current multiplication. A microchannel plate is a two-dimensional array of hollow glass fibers, fused together into a thin disk. The inside surface of the hollow glass fibers is covered by a resistive secondary electron emission film, which is electrically connected to the input and the output electrodes of the channel plate. The hollow glass fibers, generally termed microchannels, have an inside diameter in the 8- to 12 .mu.m range. The microchannels are not perpendicular to the input and output surfaces but typically are at a 5- to 10 degree bias angle. The purpose of the bias angle is to ensure a first electron impact near the channel entrance, reduce light feedback from the phosphor screen, and improve uniformity of the image transmission.
Etchable glass rods (cores) are clad with lead-silicate glass. After being drawn smaller, the clad rods are cut and fused into hexagonal array bundles. They are then drawn a second time, cut and fused into a boule, which is sliced into thin wafers, ground and polished to the final dimensions of the microchannel plate. The microchannels are obtained by etching the core glass from the lead-silicate glass structure.
The resistive secondary emission film covering the inside surface of the microchannels is obtained by hydrogen firing the MCP structure to reduce the lead-oxide glass to lead and water. The finely dispersed lead produces semiconduction in the lead oxide.
The inside surface of the microchannel electron multiplier is a continuous, resistive strip. By impressing a voltage across the microchannel, a homogeneous, axially-oriented electric field is produced in the channel. A primary electron, striking the input end of the channel, produces a multiple number of secondary electrons. The secondary electrons enter the axial electric field with a small, initial component of transverse velocity, causing the electrons to move on a parabolic path along the length of the channel until they collide with the channel wall again and generate more secondary electrons. The multiplication process continues until the end of the channel is reached.
If the electroding is extended into the channel at the output end, typically to a depth of one to three channel diameters, some collimation can be achieved. This process has been shown to improve resolution. It also destroys secondary emission where the electroding covers the walls, reducing the effective gain of the MCP by a few percent. End spoiling will not be necessary if a collimator is used near the screen.
MCPIs are the most significant element limiting the resolution of streak cameras. At present, the only method of increasing resolution for these applications is to use a larger diameter intensifier. This is a possible, though expensive, solution only for systems using 18 or 25 mm intensifiers, since 40 mm tubes are the largest available and cost about three times as much as 18 mm tubes.
It would be advantageous to prevent elastically-scattered electrons and electrons with high transverse energy from reaching the screen. This would improve dynamic range and spatial resolution of MCPIs.